Note that a significant portion of wind generation output occurs at night, when the energy has relatively low value. And, in some cases there is so much “baseload” generation – whose output cannot be varied – plus wind generation that system operators must either shut down (“curtail”) wind generation or pay other utilities, businesses or residential end-users to take the excess energy – sometimes referred to as “negative pricing.”
With storage that “off-peak” energy from wind generation can be stored and used during the day, reducing:
- The need for generation capacity (equipment), and
- Operation during peak demand periods (thereby reducing air emissions, fuel use, maintenance and wear related costs).
Storage could do the same with energy output from RE generation that does have a constant output, especially biomass and geothermal fueled generation.
For solar, storage could be charged (filled) at night with low-cost electricity from the grid,and the energy could be used duringpeak demand time, — usually hot summer week day afternoons – when the solar generation is falling off for the day. Perhaps less desirable, but still interesting, is the possibility of time-shifting the energy produced in the morning, before peak demand times (in the afternoon) so that energy is available to serve peak demand as the solar generation is falling off.
To address longer-duration variability throughout agiven day, storage can be used to provide power when two conditions exists:
- RE generation power output is less that its full power output, and
- When electricity demand is high.
To do that, the storage discharges to “fill-in” when the RE generation is not producing full power. The effect – of storage used in in concert with variable RE to address daily variability – is what is sometimes called “firming,” meaning that the result is constant power output, especially during times of peak demand.
Energy Storage to Address Renewable Generation Forecasting Uncertainty
One challenge associated with RE generation is that weather-related forecasts are imprecise. So there is always some uncertainty about prevailing weather conditions – wind speed and cloudiness – when RE generation is expected. Importantly, this challenge is being addressed and forecasting will improve. For example, there is a significant interest in improving “hour-ahead” forecasts rather than relying heavily on “day-ahead” forecasts.
Nonetheless there will always be some uncertainty about the weather. And the results can be somewhat dramatic, especially for wind generation because wind can be somewhat to very different than forecasts would indicate, especially when the forecast is a day-ahead.
When unexpected shortfalls of RE generation output occur some other resource is needed to make up the difference. Storage could be a valuable and flexible resource for addressing this challenge by providing “back-up” when RE generation is significantly less than expected.
Energy Storage to Address Power Quality Effects From Renewable Generation
Storage can also be used to offset undesirable effects on power quality (PQ) at and near points where problematicRE generation is connected to the grid. Of special note are parts of the distribution system where there is a significant penetration– 30% of peak demand or higher – of distributed photovoltaic (PV) systems. In parts of the distribution system with such high penetration of distributed PV, significant voltage variability may occur, affecting the overall electric service quality.
In some cases, the distributed photovoltaics produce an amount of power that exceeds the local demand. In those situations the excess electricity must be transferred to other sections of the distribution systems, or even to other regions. Unfortunately, that requires electricity “back-flow” (from lower voltage to higher voltage) through equipment that is designed for one way electricity flow (higher voltage to lower voltage). Under some conditions, wind generation can also cause electrical problems.
Storage can be used to address many PQ related challenges from RE generation by absorbing, filtering or otherwise offsetting many power quality anomalies. For example, when the voltage on a local distribution system varies too much, the storage can absorb excess power and even some types of abnormal power, such as current surges. Storage can also provide “real power” or “reactive power” to improve the voltage. Storage can also absorb excess power to reduce undesirable back-flow. Much larger storage could do the same for the transmission system in locations where many large wind turbines are installed.
Conclusions & Observations
Energy storage and variable RE generation have important synergies, making them quite complementary. Energy storage power input and output can be controlled precisely and varied rapidly to offset effects from RE generation short duration variability. Storage can store RE generation output when that output has low value for use later when the energy has higher value (energy time-shift). Storage can also be used to firm power output from variable RE generation during times when electricity demand is high. The resulting benefits include:
Reduced need for conventional generation,
Increased value of the electricity generated by variable RE generation,
Reduced suboptimal operation of conventional generation due to reduced need for ramping, and
Reduced need for other power conditioning equipment to accommodate large wind farms and high penetrations of distributed photovoltaics.
Given the aforementioned synergies and benefits, it is quite reasonable to assume that energy storage is likely to play a significant role in the more sustainable, cleaner, variable and distributed electricity grid future by:
Offsetting negative effects on the grid from RE generation variability,
Increasing the value of variable RE generation output, and
Enabling more variable RE generation deployment.